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I learned about gas laws and their ability to compress. My science teacher told me that solids and liquids are incompressible. But when I learned about nuclear fission in bombs, it talks about compressing the uranium. How is this so?

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  • $\begingroup$ Solids and liquids are compressible, just not as much as gasses so high school physics will usually assume them to be incomprehensible to make math easier. In the case of nuclear bombs there is a whole lot more going on involving relativity. The atoms inside the uranium essentially lose cohesion and turn into a liquid of sorts. If you are truly interested I am happy to explain, it will just take a long time. $\endgroup$ – Sponge Bob Aug 27 '15 at 5:02
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In thermodynamics there is the compressibility \begin{equation} \kappa = - \frac{1}{V_\text{mol}}\left( \frac{\partial V_\text{mol}}{\partial p} \right)_T \end{equation} in order to describe how well one can compress a substance. For small (!) changes of pressure $\Delta p$ (while you keep temperature constant), $\Delta p \kappa$ tells you by which percentage the molar volume $V_\text{mol}$ is reduced.

Ideal gases have a compressibility of $\kappa = \frac{1}{p}$, so at standard pressure, $p_0 = 1.01325$ bar, you need a pressure change of roughly $\Delta p = 0.0101325$ bar in order to compress an ideal gas by 1%. This behavior is usually called "compressible".

When your teacher told you that solids and liquids are incompressible he was only a little bit imprecise, since these substances are not incompressible but rather have a very small compressibility. For example Wikipedia lists the compressibility of water as being $\kappa = 4.6 \times 10^{-10} \text{Pa}^{-1}$ at 25 °C.

So you can very well compress liquids and solids (such as uranium), but you need very high pressures for that.

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In a nuclear fission, the heavy,unstable and radioactive uranium nucleus is hit by a fast moving neutron having kinetic energy in excess of 5 MeV. The process continues with subsequent emission of highly-energised neutrons and other fragments. This suggests that a very high temperature and a high pressure is required for sustaining the process. So, what happens is that at such high temperature, the uranium atoms in the fission sample get thoroughly excited and the sample almost reaches some kind of a molten state, attaining some compressibility. Moreover, the fast moving neutrons on account of their high velocities are able to collide with or in your words compress the U-238, U-235 nuclei successfully and initiate nuclear fission. Also solids and liquids do have compressibility, but that is very low.

So I think that this behavior of uranium during fission is what you mean by compressing the uranium.

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